This invention relates to impinging jet air driers for use in the drying of large sheet material, such as strips or sheets of veneer and, more particularly, to the configuration of the jet drier tubes which serve to impinge heated air onto the sheet material as the same travels through the drier.
The design of jet drier tubes used in impinging jet air driers has posed a difficult problem for a long time. A variety of configurations have been tried, but none has been completely satisfactory. Among critical factors involved in the design are the cross-sectional shape of the tube itself, the size, shape and distribution of the jet orifices or apertures, and the spacing or distance of the tubes above and below the plane of travel of the sheet material as it passes through the drier.
A variety of cross-sectional configurations have been tried. Morris U.S. Pat. No. 3,418,727 and Holden et al. U.S. Pat. No. 3,474,544 disclose tubes with square cross-sections, but since the longitudinal axes of the tubes are disposed perpendicular to the direction of travel of the wet or "green" veneer as it passes through the drier, the upstream corner of a square tube tends to cause the often curled leading edge of the veneer strip to hang-up as it passes by, thereby causing a jam and the operation to be stopped. To obviate the problem, it has been necessary to space the tubes or boxes an undesirably large distance away from the sheet material, thereby to lose drier efficiency. Morris U.S. Pat. No. 3,314,164 discloses an attempt to remedy this deficiency by providing a trapezoidal cross-section, with the upstream edge of the box spaced further away from the sheet material. This configuration, however, sacrifices the ability of the drier box to function as a veneer guide on the upstream side thereof.
Stock U.S. Pat. No. 3,324,571 disloses jet tubes of circular cross-section. While this configuration avoids any hang-up problem on the upstream side, it fails to provide any guiding function on the downstream side, thereby to allow the sheet veneer to curl up between the tube and the next pair of drive rolls, thereby also to cause work stoppages.
The configuration of the actual orifice itself has also posed a problem. Morris U.S. Pat. No. 3,314,164 and Stock U.S. Pat. No. 3,324,571 disclose drier tubes having recessed orifices comprising elongated slots with inwardly extending flanges. Recessing the orifice, however, often causes the sheet veneer to get caught up in the discharge slot, also to cause a plug-up and work stoppage.
The use of multiple apertures has been disclosed as a way of improving drying efficiency. In "Plywood Manufacturing Practices," by Richard F. Baldwin, published 1975 by Miller Freeman Publications, Inc., San Francisco, Calif., page 153, it is stated that three rows of strategically located openings on the face of a jet tube more efficiently scrub the veneer and transfer the heat than do conventional one-row openings. Comstock U.S. Pat. No. 3,739,490 discloses such a plurality of orifices spaced in a particular pattern on the box face, but the tube or box configuration is rectangular in cross-section, tapering longitudinally, and does not avoid the hang-up problems at the upstream edge as above discussed.
It is thus the primary object of the present invention to provide an improved configuration for a jet drier tube or box, such that the tube is shaped and can be spaced sufficiently close to the sheet material to serve as a guide on both the upstream and downstream sides thereof.
It is a further object of the present invention to provide such a tube or box with an orifice or aperture pattern that will provide improved and efficient transfer of heat to the veneer.
It is a still further object of the present invention to provide such a tube with orifices or apertures of a type that will obviate the possibility of the sheet material plugging up the orifices when the tubes are positioned very close to the traveling sheet material.
It is a still further object of the present invention to provide a jet drier tube that will have clean, smooth lines throughout the zone where the veneer passes through, and that can discharge an increased volume of air over a wider area of the veneer than has been heretofore possible.